I noticed you didn't mention the gameplay, which I find great fun, but yes, SA1 onwards basically tries to introduce dramatic plots to the series. The characters, as well, are very preference - based. I quite like all of it, but I can't blame you for not liking the suddenly SOOPER SRS CHAOS EMERALD SPLODES EARTH plot or emo the hedgehog.
Gameplay I'm mixed on. I actually quite enjoyed Sonic Adventure's gameplay, at least at first. It was pretty fun to run around in the open areas as Sonic. I honestly don't remember how Knuckles and Tails played. Did they introduce the emerald finding gameplay in SA1? I didn't much care for that, it was jarring compared to playing as Sonic. I didn't care much for playing as Amy, and outright hated playing as Big. E-102 was alright, if strange for a Sonic game.
The
problem, was that the game was kind of buggy and it was
way too darn easy to get yourself killed trying to enjoy running around at high speed. Homing attack was also the source of many a death.
All in all, I'd give the gameplay 8/10 for Sonic, and probably 6/10 for all of the characters averaged together. As a whole, I'd probably give the game 6.5/10 to maybe 7/10. It doesn't have a lot of the problems later games do, but it does still have problems.
To me, Sonic Adventure 2 was all of the problems of Sonic Adventure (and arguably worse for gameplay), plus characters and a plot I couldn't get into. I still loathe Rouge as one of the single most out of place characters in the entire series. Don't get me started on the transformer plane Tails drives. I wanted to play Tails, not
that. I can tolerate Shadow, but he should have stayed dead. Coming back kind of destroyed his character development to me.
Sonic Heroes had awful gameplay. The bugs were even
worse. I fell through the world geometry 5 seconds into the game. The flying characters were almost pointless, and I only ever switched to power because I
had to to get through an area. Bosses were very frustrating, especially the Egg Emperor (was that the name?). I was also quite disappointed that I didn't get to fight Metal Sonic. Instead, I got to fight Metal Godzilla.
I don't know. Maybe Sega got their act together for Generations or Sonic 4. Maybe I should actually try one of them. I'm not touching Sonic 2006 though.
Well, that's one rant out of the way. I've got several more I could spew out instead of working like I should be.